Developing Nanocomposites for Isotropic Selective Laser Sintering

Selective Laser Sintering (SLS) is currently one the most popular additive manufacturing (AM) technologies for the fabrication of end-use parts. SLS applications are constantly increasing across multiple industries going from the fabrication of consumer products like custom cases for smartphones to custom parts for military aircraft and even sophisticated implantable medical devices. The layer-by-layer technology that pioneered rapid prototyping applications is rapidly evolving into a rapid manufacturing mature process.

Selective Laser Sintering (SLS) is currently one the most popular additive manufacturing (AM) technologies for the fabrication of end-use parts. SLS applications are constantly increasing across multiple industries going from the fabrication of consumer products like custom cases for smartphones to custom parts for military aircraft and even sophisticated implantable medical devices. The layer-by-layer technology that pioneered rapid prototyping applications is rapidly evolving into a rapid manufacturing mature process.

While the number of SLS applications is constantly growing, the process' material limitations have remained tolerably unchanged for the past 12 years. The new generation of aerospace engineers, designers, and freeform manufacturing advocates still face the challenge of designing products for an anisotropic process. As a rule of thumb the average anisotropy for SLS unfilled nylon 12 polymers is about 5-8%. The anisotropy of the same polymers with micron size fillers jumps to about 20-40%. This process characteristic slows the rate of technology adoption and delays its transition into large-volume part manufacturing programs; hence tolerance for this type of barrier is no longer acceptable.

3D Systems Corporation through its Paramount group in Langhorne, PA has developed a state of the art R&D facility that is currently addressing this challenge. For the first time in the AM industry SLS is successfully advancing towards isotropic and superior material properties. The innovation comes from the research and development of advanced SLS thermoplastic nanocomposites that have the capability to exceed the mechanical properties of traditional SLS materials (e.g. DuraformPA and DuraformEX).

Due to the variable size of the nanoparticles a material selection and design method must be performed to select the polymer matrix, the nanoparticle type, and the dispersion method that better suits a given application. During this selection process one must not forget the health and safety concerns that come with handling SLS powders with fillers in the nanoscale. For this application we compared the benefits of multiwall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) and decided to use carbon nanofibers as the current equipment technology and SLS manufacturing environments are not suited to safely handle MWCNTs. Figure 1a-b compares the particle size of MWCNT and CNFs.

Figure 1. a. SEM of MWCNTs, Mag. 20KX. b. Vapor-Grown CNFs, Mag. 20KX

The success of a nanocomposite for SLS is highly dependent on the overall product quality control and process control of the nanoparticles and the dispersion method utilized. Multiple dispersion techniques can yield significantly different results as shown in Figures2a-b. If the dispersion techniques fail to exfoliate the nanoparticles, these will tend to re-agglomarate during processing, damage the aspect ratio of the nanoparticle or have a non-uniform distribution in the end product as in the case of the carbon nanofibers shown in Figure 2a.

The dispersion methods are extremely important to obtain the full advantage of the mechanical properties of nanoparticles. Inconsistent dispersion ratios between particles will translate into inconsistent mechanical properties between laser sintered parts. The resulting microstructure of a poorly blended carbon nanofiber composite will have regions lacking microstructural reinforcement (Figure 3).

After properly dispersing the CNFs the resulting sintered matrix shows an even distribution. Nanocomposites where the dispersion of carbon nanofiber is evenly distributed exhibit a crossection with uniform distribution of the CNFs throughout the sintered polymer matrix (Figure 4a-b). The results indicate that CNFs are randomly oriented throughout the polymer matrix in the X-Y-Z planes. This achievement represents a remarkable process improvement from traditional SLS materials reinforced with carbon fibers where the fibers get randomly oriented but only on the X-Y plane. In the SLS process the z-axis is considered the “weak axis” due to the layerwise fabrication method. Therefore, fibers oriented in all directions significantly improve the mechanical properties of parts in their z-axis and reduce the process induced anisotropic characteristics of a final part.

In the case of nylon nanocomposites the uniform distribution of the CNFs may not be as good using compounding methods since previous studies have shown that CNTs and CNFs tend to agglomerate around the crystalline regions of the polymer during compounding. This phenomenon would make the dispersion of the CNFs strongly dependent on the crystallization behavior of the polymer matrix and in turn during sintering it would form heterogeneous nucleation sites where crystallization would occur faster and earlier. Some compounding techniques, however, have also shown promising results addressing this phenomenon.

The use of nanoparticles provides the flexibility of engineering materials for specific applications. While some applications may be focused on mechanical properties, others may require chemical resistance, electrical conductivity, and high temperature performance. In this particular application the requirement was isotropic mechanical properties including improved elongation at break. Figures 5-6 show the tensile test results for a nylon-12/CNF laser sintered nanocomposite compared against the tensile properties for one of the leading nylon-12/Carbon Fiber SLS materials (Tensile specimens for both materials where fabricated utilizing the same laser sintering machine).

These results show the ability to achieve uniform mechanical properties utilizing CNF-reinforced nylon-12. The improved material performance out of this layerwise process is attributed to the random orientation of the CNF throughout the polymer matrix. The same performance is not possible utilizing micronized carbon fibers as they tend to orient primarily in the XY plane.

Environmental, Health, and Safety Issues

Like many new products today, nanoparticles are being investigated for their Environmental, Healthy, and Safety (EH&S) impacts. Since the technology is still rather new, there have not been any definitive studies on these materials, but there have been numerous studies to date showing that there are issues of concern. The main issue with nanocomposite parts and goods comes down to worker exposure during manufacture of the polymer nanocomposite, and consumer exposure post-manufacture due to normal wear and tear of the composite part.

In regards to the SLS propocessing of nanocomposites, the concerns on worker exposure come from both exposure to raw nanoparticles during nanocomposite loading, break-out, and clean-up operations, and exposure to nanoparticles from the nanocomposite during drilling/sanding/machining operations. Exposure to nanoparticles during SLS can be in form of inhalation or skin/eye exposure when dry nanoparticles are released during handling.

In studies conducted by the National Institute of Occupational Safety and Health (NIOSH), it was found that most of these exposure issues can be addressed with engineering controls as well as appropriate personal protective equipment (PPE). Specifically, if dry nanoparticles must be used, fume hoods plus HEPA or ULPA-type filter containing respirators must be used to prevent exposure, and preferably, the nanoparticles should be handled in a wet state (wetted out with polymer, solvent, etc.) rather than handled dry. If they must be handled dry, secondary containment is highly recommended as well as additional particulate capture systems throughout the lab (Figure 7).

Exposure to nanoparticles during post nanocomposite synthesis machining operations (cutting, sanding, drilling, etc.) depends more upon how the nanoparticle interacts with the polymer matrix and what size particulates are generated from the machining operation. If the nanoparticle is thoroughly bound/covered in polymer, some initial data from the University of Dayton Research Institute (UDRI) suggests that the nanoparticle will remain in the polymer and the primary exposure issue is polymer dust, not nanoparticles. However, if the nanoparticles are not well encapsulated by polymer then machining operations could release these nanoparticles. For this reason it is particularly important that a good polymer nanocomposite part should have even dispersion of the nanoparticles fully wetted out by polymer to have acceptable commercial properties. Once we can prove the repeatability of a good dispersion and wetting by the polymer matrix this risk may not be relevant when true commercial parts are produced and being used, but more studies are needed.

The researcher would like to acknowledge the support provided for this study by Mr. Tim Gornet from University of Louisville, Dr. Alex Morgan and Dr. Brian Rice from the University of Dayton Research Institue, and Mr. Arthur Fritz from Nanosperce LLC.